Each XA statement begins with the XA keyword,
and most of them require an xid
value. An xid is an XA transaction
identifier. It indicates which transaction the statement applies
to. xid values are supplied by the
client, or generated by the MySQL server. An
xid value has from one to three
parts:

xid: gtrid [, bqual [, formatID ]]

gtrid is a global transaction
identifier, bqual is a branch
qualifier, and formatID is a number
that identifies the format used by the
gtrid and
bqual values. As indicated by the
syntax, bqual and
formatID are optional. The default
bqual value is ''
if not given. The default formatID
value is 1 if not given.

gtrid and
bqual must be string literals, each
up to 64 bytes (not characters) long.
gtrid and
bqual can be specified in several
ways. You can use a quoted string ('ab'), hex
string (0x6162, X'ab'), or
bit value
(b'nnnn').

formatID is an unsigned integer.

The gtrid and
bqual values are interpreted in bytes
by the MySQL server's underlying XA support routines. However,
while an SQL statement containing an XA statement is being
parsed, the server works with some specific character set. To be
safe, write gtrid and
bqual as hex strings.

xid values typically are generated by
the Transaction Manager. Values generated by one TM must be
different from values generated by other TMs. A given TM must be
able to recognize its own xid values
in a list of values returned by the
XA
RECOVER statement.

XA START xid
starts an XA transaction with the given
xid value. Each XA transaction must
have a unique xid value, so the value
must not currently be used by another XA transaction. Uniqueness
is assessed using the gtrid and
bqual values. All following XA
statements for the XA transaction must be specified using the
same xid value as that given in the
XA
START statement. If you use any of those statements
but specify an xid value that does
not correspond to some existing XA transaction, an error occurs.

One or more XA transactions can be part of the same global
transaction. All XA transactions within a given global
transaction must use the same gtrid
value in the xid value. For this
reason, gtrid values must be globally
unique so that there is no ambiguity about which global
transaction a given XA transaction is part of. The
bqual part of the
xid value must be different for each
XA transaction within a global transaction. (The requirement
that bqual values be different is a
limitation of the current MySQL XA implementation. It is not
part of the XA specification.)

The XA
RECOVER statement returns information for those XA
transactions on the MySQL server that are in the
PREPARED state. (See
Section 12.3.7.2, “XA Transaction States”.) The output includes a row for each
such XA transaction on the server, regardless of which client
started it.

XA
RECOVER output rows look like this (for an example
xid value consisting of the parts
'abc', 'def', and
7):